The present invention relates generally to communication systems, and more particularly to interference mitigation in communication systems.
The structure and operation of high-speed data communication systems are generally known. Such high-speed data communication systems employ various media and/or wireless links to support the transmission of high-speed data communications. Particular embodiments of high-speed communication systems include, for example, cable modem systems, home networking systems, wired local area networks, wired wide area networks, wireless local area networks, satellite networks, etc. Each of these high-speed data communication systems has some unique operational characteristics. Further, some of these high-speed data communication systems share similar operational drawbacks. Home networking systems and cable modem systems, for example, are both subject to interfering signals that are coupled on media that carry communication signals.
Cable modem systems, and more generally, cable telecommunication systems include set-top boxes and residential gateways that are in combination capable of currently providing data rates as high as 56 Mbps, and are thus suitable for high-speed file transfer, video teleconferencing and pay-per-view television. These cable telecommunication systems may simultaneously provide high-speed Internet access, digital television (such as pay-per-view) and digital telephony. Such a system is shown and described in U.S. patent application Ser. No. 09/710,238 (Attorney Docket No. 34690), entitled xe2x80x9cPre-Equalization Technique for Upstream Communication Between Cable Modem and Headendxe2x80x9d, filed on Nov. 9, 2000, the disclosure of which is expressly incorporated by reference.
Cable modems are used in a shared access environment in which subscribers compete for bandwidth that is supported by shared coaxial cables. During normal operations, sufficient bandwidth is available across the shared coaxial cables to service a large number of subscribers, with each subscriber being serviced at a high data rate. Thus, during normal operations, each subscriber is provided a data rate that is sufficient to service uninterrupted video teleconferencing, pay-per-view television, and other high bandwidth services.
Intermittent, narrowband interfering signals may, from time to time, interfere with wideband communication signals, e.g., upstream Data-Over-Cable Service Interface Specification (DOCSIS) transmissions (xe2x80x9cdesired signalsxe2x80x9d). These intermittent narrowband interfering signals unintentionally couple to the shared coaxial cables via deficiencies in shielding and/or other coupling paths. With these interfering signals present, the data rate that is supportable on the coaxial cables is reduced. In some cases, depending upon the strength and band of the interfering signals, the supportable bandwidth is reduced by a significant level.
Conventionally, when an interfering signal is present, an adaptive cancellation filter is employed by each cable modem receiver to cancel the interfering signal by adaptively placing a filtering notch or null at the frequency of the interfering signal. When the interfering signal becomes absent, the conventional adaptive cancellation filter continues to adapt and removes the filtering notch. If the interfering signal reappears, the adaptive cancellation filter again adapts to null the interfering signal. Thus, when the interfering signal first reappears, the cancellation filter cannot fully compensate for the interfering signal. Because many interfering signals are intermittent, the presence of these intermittent signals reduces the bandwidth that is supportable upon the coaxial media during the time period required for the cancellation filter to adapt. Further, because the interfering signals oftentimes vary in strength while present, the adaptive cancellation filter most often times does not fully remove the interfering signal.
Overlapping adjacent channel signals cause another source of interference for the desired signal because they often produce interfering signals in the band of the desired signal. For example, a TDMA signal that resides in an adjacent channel and that turns on and off may have side lobes that overlap and interfere with the desired signal. When the interfering signal is present, the conventional adaptive cancellation filter places a notch or null at the frequency band of the interfering signal. When the interfering signal is absent, the cancellation filter adapts to removes the notch. The precise amount of interference in the desired signal caused by the adjacent channel signals may vary with data content in the adjacent channel.
Thus, in both the case of the narrowband interferer and the adjacent channel interferer, the interfering signal(s) varies over time. For this reason, an optimal or near-optimal solution may be found only for the average interfering strength of the interfering signal(s), but not for the peak(s) of the interfering signal(s). In many operational conditions, typical fluctuations in the strength of interfering signals cause conventional cancellation filters to provide insufficient cancellation. Resultantly, overall bandwidth that could be provided by the supporting communication system on the particular shared media is significantly reduced.
Therefore, there is a need in the art for a filtering system and associated operations that cancel interfering signals so that throughput is maximized.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art to efficiently filter a communication channel to remove narrow band interfering signals. According to the present invention, a communication channel is sampled to produce a sampled signal. Then, the sampled signal is spectrally characterized across a frequency band of interest to produce a spectral characterization of the sampled signal. This spectral characterization may not include the signal of interest. The spectral characterization is then modified to produce a modified spectral characterization. Filter settings are then generated based upon the modified spectral characterization. Finally, the communication channel is filtered using the filter settings when the signal of interest is present on the communication channel.
Various operations are employed to modify the spectral characterization to produce the modified spectral characterization. In most cases, a plurality of spectral characteristics of the spectral characterization are independently modified to produce the modified spectral characterization. Modifications to the spectral characterization may be performed in the frequency domain and/or the time domain. One particular spectral modification that is performed is raising of the noise floor of the spectral characterization to meet a budgeted signal-to-noise ratio.
Other spectral modifications include modifying spectral components corresponding to an expected interfering signal. In modifying these spectral characterizations, spectral components corresponding to a plurality of expected interfering signals may be modified. In obtaining information to perform this spectral modification, spectral components of prior sampled signals may be employed. For example, an interfering signal may be intermittent so that it is not present in the current sampled signal but was present in prior sampled signals. Since overall system performance may be enhanced when the filter compensates for this interfering signal even when it is not present, in modifying the spectral characterization of the sampled signal to produce the modified spectral characterization, the presence of an interfering signal in a prior sampled signal may be weighted more heavily than the absence of the interfering signal in the current sampled signal.
The communication channel may be sampled with or without the presence of the signal of interest. However, when the signal of interest is present on the communication channel during the sampling interval, the signal of interest must be removed from the sampled signal. Further, when the total spectral density of the sampled signal exceeds a threshold value, the sampled signal may be discarded as invalid.
Because the filtering operations of the present invention adapt to time varying interfering signals in an adaptive fashion, more efficient filtering operations are performed. For example, by raising the noise floor of the spectral characterization, budgeted SNR design constraints may be met with minimal filter tap magnitudes. Further, by enabling the introduction of known interferers into the modified spectral characterization, the filtering operations will compensate for interfering signals immediately when they appear, not after a learning period of the prior filtering operations. Because of these benefits, among others, greater system throughput is realized.
Other features and advantages of the present invention will become apparent from the following detailed description of the invention made with reference to the accompanying drawings.